The present invention relates to nuclear magnetic resonance (NMR) apparatus and, more particularly, to novel NMR antenna probes utilizing a plurality of surface coils, (i.e. a conductor or a plurality of conductors, with or without added impedance elements, which are arranged and substantially confined initially to a flat plane, even if that flat plane is later to be conformed to the shape of a sample of interest, for actual use in exciting and/or detecting/receiving NMR signals), arranged in substantially coaxial and coplanar geometry and having improved electronic isolation therebetween.
It is well known to utilize surface coils as NMR reception (or detection) antennae, particularly for spectroscopic studies of the head, torso and the like, because such coils provide a better signal-to-noise ratio than volume coils. Surface coils are also commonly used for excitation in spectroscopic studies because the excitation field is more localized, so that less radio-frequency (RF) power is required. Due to increased bandwidth requirements, the RF power required to stimulate NMR nuclei other than hydrogen (.sup.1 H) may become excessive; in some cases, this leads to undesirable RF power levels in the sample (e.g. a human body portion) under study, as nuclei such as phosphorus (.sup.31 P) or carbon (.sup.13 C) are studied. It is highly desirable to utilize simple circular or polygonal surface coils for reception, as these surface coils are most sensitive in such studies. It is often desirable to utilize separate surface coils for transmission of the excitation signal and for reception of the NMR response signal. Situations in which separate transmission/reception surface coils may be used include: heteronuclear NMR studies, where one coil is used for exciting a first species of nucleus, and a second coil is used for exciting and/or detecting a second species of nucleus, as in a .sup.1 H-decoupled, nuclear Overhauser enhanced, in vivo .sup.13 C experiment, or in a .sup.1 -decoupled in vivo .sup.31 P experiment; in a heteronuclear NMR study where a first coil is tuned and used for excitation of .sup.1 H images and a second coil is used for excitation and/or detection of NMR spectroscopy signals from the sample; in a homonuclear study utilizing selective irradiation of one chemical species with a first coil and utilizing a second coil for excitation and/or detection of NMR signals from at least one chemically different species of the same NMR nucleus; or a homonuclear study where a first coil is used for excitation (e.g. to provide a uniform excitation field over a region of a sample), and a second coil is utilized for detection. The foregoing experiment types encompass the vast majority of NMR spectroscopy studies performed at this time.
It is also well known that use of a pair of circular or polygonal surface coils, positioned to produce fields that are substantially coaxial with one another, provide a degree of mutual electrical coupling between the coils which seriously compromises performance of the probe utilizing the pair of coils. Often, the performance of the higher-frequency coil (i.e. that surface coil tuned to the higher NMR frequency, e.g. to the .sup.1 H frequency) is rendered useless, and diode-blocking or other active blocking networks cannot be employed for decoupling studies. Thus, in .sup.1 Hdecoupled spectroscopy, where a high level decoupling signal must be provided during the time interval when spectra are actually acquired, the mutual interaction between the decoupling coil and spectra-reception coils will prevent acquisition of the NMR response signal by the latter (detection) coil. It is therefore highly desirable to provide a NMR probe antenna, having a plurality of surface coils, in which at least a pair of the surface coils do not substantially interact, even though those coils are substantially coplanar and coaxial with one another.